Conveying arrangement

ABSTRACT

A conveying arrangement for conveying articles from a lower to a higher level. An upright frame has a lower terminal station and an upper terminal station and at least one intermediate station which is located between and equally spaced from both terminal stations by a first distance. A lift arrangement is guided in the frame for alternate movement in a predetermined path between a lower and an upper position by a distance which is slightly greater than the first distance. The lift arrangement has a lower and at least one upper transfer level which in the lower position are respectively located slightly below the lower terminal station and the intermediate station, and which in the upper position are respectively located slightly above the intermediate station and the upper terminal station. First detent means is provided at the respective transfer levels for supporting articles to be conveyed as the lift arrangement moves to its upper position. Second detent means is provided at the respective stations of the frame and permits upward movement of articles supported by the first detent means but intercepts and retains such articles at the respective stations when the lift moves to its lower position. Moving means moves the lift between the positions thereof.

United States Patent Reger et al.

[ 1 Oct. 17,1972

[54] CONVEYING ARRANGEMENT [72] Inventors: Herbert Reger, Moglingen; Hans- Walter Heinrich, Ochtersum, both of Germany [73] Assignee: Robert Bosch GmbH, Stuttgart, Germany [22] Filed: v Sept.29, 1970 [21] Appl.No.: 76,363

[30] Foreign Application Priority Data Primary Examiner-Gerald M. Forlenza Assistant Examiner-Lawrence J. Oresky AttorneyMichael S. Striker [5 7] ABSTRACT A conveying arrangement for conveying articles from a lower to a higher level. An upright frame has a lower terminal station and an upper terminal station and at least one intermediate station which is located between and equally spaced from both terminal stations by a first distance. A lift arrangement is guided in the frame for alternate movement in a predetermined path between a lower and an upper position by a distance which is slightly greater than the first distance. The lift arrangement has a lower and at least one upper transfer level which in the lower position are respectively located slightly below the lower terminal station and the intermediate station, and which in the upper position are respectively located slightly above the intermediate station and the upper terminal station. First detent means is provided at the respective transfer levels for supporting articles to be conveyed as the lift arrangement moves to its upper position. Second detent means is provided at the respective stations of the frame and permits upward movement of articles supported by the first detent means but intercepts and retains such articles at the respective stations when the lift moves to its lower position. Moving means moves the lift between the positions thereof.

' 8 Claims, 4 Drawing Figures CONVEYING ARRANGEMENT BACKGROUND OF THE INVENTION Still more specifically the invention relates to a con- 1 veying arrangement for conveying workpiece supports of plate-shaped configuration from the level of a lower supply conveyor to the level of an upper substantially horizontal removal conveyor.

Conveying arrangements of this general type are already known. For instance, one such conveying arrangement utilizes a frame-shaped lift or receiver having means for engaging and lifting one workpiece support at a time. In this construction the workpiece support is moved off the lower supply conveyor into or onto the lift arrangement at the lower terminal station of the construction, and thereupon the lift arrangement with the workpiece support is lifted in a single stroke from the level of the lower terminal station to the level of the upper terminal station where the workpiece is discharged from the lift arrangement by a suitable device and placed onto the upper removal conveyor.

-To repeat this performance it is necessary for the lift arrangement to descend from the level of the upper terminal station to the level of the lower terminal station and only now loading of a new workpiece support into the lift arrangement can begin.

It follows from this that the time period required for completion of one conveying episode, that is the time from the moment at which a workpiece carrier is loaded into the lift arrangement to the time the next carrier can be loaded into the lift arrangement after the first one is discharged, is thus the sum of the times required for moving the workpiece support off the lower supply conveyor into the lift arrangement at the lower terminal station, withdrawing the device effecting the movement, lifting the lift arrangement from the level of the lower terminal station to the level of the upper terminal station, moving the lifted workpiece support out of the lift arrangement onto the removal conveyor, withdrawing the device used for effecting such movement, and lowering the lift arrangement to the level of the lower terminal station. It follows that the time period required for the lifting or conveying of each individual article, such as a workpiece support, from the lower terminal station to the upper terminal station is relatively long which is a decided disadvantage in many applications.

SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to overcome this aforementioned disadvantage.

More particularly, it is an object of the present invention to provide an improved conveying arrangement of the general type under discussion which substantially decreases the conveying time for each individual article.

Another object of the invention is to provide such an improved conveying arrangement which not only decreases the conveying time required for conveying of each individual article, but which effects such decrease without increasing the speed of conveyance itself.

An additional object of the invention is to provide such an arrangement which is relatively simple in its construction and therefore comparatively inexpensive to produce.

In pursuance of the above objects, and others which will become apparent hereafter, one feature of the invention resides in a conveying arrangement of the type under discussion which, briefly stated, comprises upright frame means having a lower terminal station, an upper terminal station, and at least one intermediate station between and equally spaced from both the terminal stations by a first distance. Lift means is guided in the frame means for alternate movement in a predetermined path between a lower position and an upper position by a second distance which is slightly greater than the first distance; this lift means has one lower and at least one upper transfer level which in the lower position are respectively located slightly below the lower terminal station and the intermediate station and which, in the upper position, are respectively located slightly above the intermediate station and the upper terminal station. First detent means is provided at the respective transfer level and arranged for supporting articles to be conveyed, during movement of the lift means to its upper position. Second detent means is provided at the respective stations and arranged for permitting upward movement of articles supported by the first detent means during movement of the lift means to its upper position, but for intercepting and retaining such articles at the respective stations in response to subsequent movement of the lift means to its lower position. Finally, moving means is provided for moving the lift means between the aforementioned positions thereof.

With a construction such as just outlined in accordance with the present invention, the time required for conveying of each individual article is substantially decreased over what is known from the art, while at the same time the speed of conveyance is not increased. This is important because an increase in the conveying speed is strictly limited by certain considerations, namely the permissible speeds of the piston arrangements preferably utilized, and the acceptable severity of the impacts which take place as the device moves to the various positions, particularly at the end of the movement of the lift means to lower position and to upper position, respectively.

Of course, there is at least one intermediate station provided between the upper and lower terminal stations, but there could be two or more such intermediate stations provided. The essence of the invention is that by resorting to the present invention conveyance of the articles can be effected by conveying or raising several articles simultaneously through distances which are a fraction of the total distance through which they are to be conveyed, namely distances which are little more than the distance between two consecutive ones of the stations. The number of articles which can be conveyed simultaneously will be one less than the total number of stations involved. Furthermore, operation of the devices which move the articles off the supply conveyor into the lower terminal station, and off the upper terminal station onto the removal conveyor, can be effected simultaneously so that, if the arrangement is for instance provided with an upper and a lower terminal station and with three intermediate stations, the stroke or distance through which each of several articles must be lifted, and the time required for such lifting, can be reduced to little more than a quarter of the total stroke required and little more than a quarter of the total time required for carrying out the total stroke. Time for separate loading of articles into the lower terminal station and unloading from the upper terminal station, and time for retraction of the devices used for this purpose, is now no longer separately required and is thus saved.

By providing the detent means according to the present invention, which is preferably in form of spring or otherwise biased pawl units, the articles can freely move upwardly in direction from the lower to the upper terminal station, but cannot be moved downwardly because they are intercepted by the pawl units. Thus, the articles to be conveyed are lifted during each movement of the lift means from its lower position to its upper position, from whatever station they are originally at to the next-higher station, and from this station to the next-following higher station as the lift means performs its next movement from lower position to upper position. During movement of the lift means from upper position to lower position the articles are intercepted and retained by the pawl units at the respective stations until such time as the lift means reverts again and moves to its upper position. As this takes place the articles are then moved to the nexthigher station.

The pawl units are constructed according to the present invention in form of double-armed levers which are spring-biased and which are both simple in their construction and very reliable in their operation as will be evident from a subsequent more detailed discussion of these units.

It is also possible to provide sound-dampening material at critical points where various components engage one another periodically, in order to reduce the noise level resulting from operation of the conveying arrangement. Furthermore, the invention provides for the possibility of utilizing adjusting arrangements for adjusting the positioning and operation of the detent means.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary diagrammatic perspective view illustrating one embodiment of the invention;

FIG. 2 is a perspective view on an enlarged scale showing the embodiment of FIG. 1 in more detail;

FIG. 3 is a diagrammatic section taken on line IIIIII of FIG. 2; and

FIG. 4 is a partly sectioned diagrammatic detail view on an enlarged scale showing the construction of one of the pawl units.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing now the drawing in detail and firstly FIG. 1 thereof, it will be seen that in the illustrated embodiment the articles to be conveyed are identified with reference numeral 1 and are illustrated as plate-shaped workpiece supports or the like. They are supplied at a lower level by a diagrammatically illustrated conveying device 7 which moves the articles 1 in the direction of the arrow 8. At the upper level the upwardly conveyed articles 1 are received by a similarly diagrammatically illustrated conveying device 11 which moves them away in the direction of the arrow 12 in an at least substantially horizontal path.

The conveying arrangement according to the present invention comprises an upright frame 2 and a lift arrangement 3 guided in the frame 2 for alternate upward and downward movement. The frame 2 has a lower terminal station 4 and an upper terminal station 5 between which the articles 1 are to be conveyed. Intermediate the stations 4 and 5 spaced identical distances from one another are provided intermediate stations 6, 6a and 6b. The distances between consecutive stations are always identical, that is the distance between the station 4 and the station 6, that between the stations 6 and 6a, that between the stations 6a and 6b, and that between the stations 6b and 5. Located at the level of the lower conveying device 7 is a loading device in form ofa sliding member 9 which is reciprocable in the direction of the double-headed arrow 9a by the diagrammatically illustrated cylinder and piston arrangement. It will be appreciated that as the member 9 moves forwardly with respect to the showing in FIG. 1, it will push the article 1 which is located in front of it on the conveying device 7, off the conveying device and into or onto the lower terminal station 4. The device 7 preferably operates continuously and a blocking member 10 is provided against which the articles 1 abut to be prevented from further movement with the device 7 which continues to move underneath them. An off-loading or unloading device is provided at the level of the conveying device 11 and utilizes a sliding member 13 reciprocable in the direction of the arrow 13a by the diagrammatically illustrated cylinder and piston unit. It serves for displacing upwardly conveyed articles 1 out of the upper terminal station 5 and onto the conveying device 11 for removal by the same in the direction of the arrow 12.

When the arrangement according to the present invention is to be first started up, articles 1 are manually placed into each of the intermediate stages 6, 6a and 6b as well as into the upper terminal station 5. Now additional articles 1 are supplied with the conveying device 7 and as the first one of these abuts against the blocking member 10, it triggers an impulse in suitable manner circuitry for this purpose is too well known to require detailed discussionwhich initiates a forward movement of the member 9, so that the latter pushes the leading article off the conveying device 7 into the lower terminal station 4. At the same time the upper member 13 whose cylinder and piston unit is similarly connected with and controlled by impulses originating from the blocking member 10, moves forwardly and expels the article 1 from the upper terminal station 5 onto the device 11. Both members 9 and 13 are then retracted simultaneously into their starting position.

Now the lift arrangement 3 is lifted upwardly by the diagrammatically illustrated cylinder and piston unit associated with it (identified in FIG. 2 with reference numeral 25) and this causes the article 1 located in the lower terminal station 4 to be lifted to the intermediate station 6, the article from station 6 to be moved to station 6a, the article from station 6a to be moved to station 6b and the article from station 6b to be moved into the terminal station 5. The articles have been retained at the various stations by the pawl units 14 which will be subsequently discussed in more detail with reference to FIGS. 2 and 4. Each article 1 in the various stations is thus lifted to the next-higher station, that is through a distance corresponding to the spacing between the successive stations. Subsequently the lifting arrangement 3 descends again to lower position by the same distance by which it was originally raised, and as soon as the next-following article 1 contacts the blocking member and triggers the operating impulse, the next conveying episode can begin in the same manner as already described.

FIGS. 2 and 3 show the arrangement according to the present invention in more detail. It will be seen that a base plate or support 16 is provided which in the illustrated embodiment is of rectangular outline. Mounted on it are two supporting members 15 extending in parallelism to the narrower sides of the plate 16. These constitute the lower terminal station 4. Also mounted on the plate 16 and rising normal to its general plane are four upright supports 17 of U-shaped cross-section whose bights extend in parallelism with the longitudinal sides of the plate 16. The supports 17 are of course rigidly mounted in suitable manner which is of no consequence for the purposes of the present invention; they may for instance be cross-braced or the like.

Secured to the bights of the upright supports 17 are pawl units, with four such pawl units being located at the level of each of the intermediate stations 6, 6a, 6b and the upper terminal station 5. These pawl units each comprise a bolt 18 whose axis is horizontal and extends in parallelism with the narrow sides of the plate 16 and on which there is mounted pivotably a pawl member 14. Details of these pawl units are described with respect to FIG. 4. The upper ends of the upright supports 17 are secured to an upper plate 19 which extends in the illustrated embodiment in parallelism with the lower plate 16. Of course, other possibilities will offer themselves readily to those skilled in the art. There are further provided two upright guidance rods 20 secured to the plates 16 and 19, respectively and extending in parallelism with the upright supports 17 at the opposite narrow sides of the plates 16 and 19.

The lift means 3 according to the present invention is of U-shaped outline with the opening of the U facing downwardly. It is itself made from U-profiled elements, such as U-shaped beams or the like, and has two upright arms 21 which are connected at their upper end by a bight or traverse 22. Secured to the arms 21 at four transfer levels 23, 23a, 23b and 230 are additional pawl units identical with those on the upright supports 17. Each level has four of these pawl units which comprise a bolt 18a whose axis extends in parallelism with the bolts 18. Again, a pawl member 14a is pivotably mounted on each of these bolts 18a. The lift means 3 is guided for upward and downward movement by having secured to it near the lower ends of the arms 21 respective guide sleeves 24 each of which embraces one of the guide rods 20.

A cylinder and piston unit 25 is mounted on the upper plate 19 and the axis of its piston rod extends in parallelism with the guide rods 20; the piston rod 26 of course penetrates through the upper plate 19 which is provided for this purpose with a suitable aperture or the like, and its free end is connected to the traverse 22. The cylinder and piston unit raises and lowers the lift means 3 as required. It is emphasized that the stroke performed by the lift means, that is the distance through which it is either lifted or lowered, is only slightly greater than the distance between any two consecutive ones of the stations in the frame 2 or of the transfer levels in the lift means 3.

In operation of the construction according to the present invention it will be appreciated that an article I to be upwardly conveyed is supported on the members 15 of the lower terminal station 4, whereas an additional article is supported at each of the stations 6, 6a and 6b on the respectively associated four pawl units of the same. These articles are illustrated in broken lines in FIG. 2. At this time the lift means is in its lower position as illustrated and its lowermost set of pawl members 14a is located just slightly below the upper surfaces of the support members 15 of the lower terminal station 4, with the pawls 14a of the other levels being correspondingly located just slightly below the pawls 14 of the respectively associated intermediate stations and upper terminal station. When in this position the piston rod 26 lifts the lift means 3 upwardly through its predetermined stroke corresponding to just slightly more than the distance between two consecutive stations, each set of pawl members 14a at the respective transfer level moves into contact with and raises upwardly the article located until then at the respective station of the frame 2 below which the set of pawl members 14a had just been located. In so doing the articles 1 themselves engage and deflect the pawl members 14 out of the way so that the articles can move up wardly past these pawl members 14 whereupon the latter are returned by the constantly acting biasing force to their original position. Details will be discussed with respect to FIG. 4. Having reached its upper position-at which each of the upwardly conveyed articles is now located just slightly above the pawl members 14 of that station which is directly above the station from which the respective article 1 has just been lifted-the lift means 3 reverses direction and now moves back to its lower position. As this takes place, the articles 1 are intercepted by the respective pawl members 14 and are thus retained, each at a station which is higher by one than the station from which it has just been lifted. The operation of the arrangement according to the present invention will thus be clear.

Coming now to FIG. 4 it is pointed out that this FIGURE shows the construction of a single pawl unit of the type provided on the frame 2. It is representative of all of the pawl units on the frame 2 as well as all of the pawl units on the lift means 3. In other words, the pawl units on the frame 2 are designated as the second detent means in the claims appended hereto, whereas the pawl units on the lift means 3 are designated as the first detent means. They are, however, all identical except that for purposes of better understanding the bolts and associated pawl members have been assigned different reference numerals in the preceding FIGURES.

Returning to FIG. 4 it is pointed out that the pawl member 14 (and of course correspondingly the pawl member 14a is of rhombic outline as identified with reference numeral 27. It is plate-shaped and provided with a bore so that it is pivotally mounted on the pin 18 which is a bolt or the like. The surface 27a normally faces upwardly in direction towards the upper terminal station and is in the illustrated embodiment provided with a recess in which there is accommodated a portion or member 28 preferably of synthetic plastic material, such as polyamide, which has sound-dampening capacities. Of course, the entire surface 27a could be so covered, and instead of a recess the portion 28 could be in form of a plate superimposed upon the surface 27a.

The edge face 27b of the member 27 is provided with a bore or recess 270 in which there is accommodated an expansion spring 29 surrounded in part by a sleeve 30 which extends outwardly of the bore 270. Secured to the uprights l7 (and correspondingly to the arms 21 of the lift means 3) is an abutment member 31 which is for instance welded to its supporting member. The sleeve 30 is urged against the upper end of the abutment member 31 under the pressure of the spring 29. At its lower end the abutment member 31 is provided with a tapped bore through which an adjusting screw or bolt 32 is threaded against whose head 33 the lower end portion of the member 27 abuts under the pressure of the spring 29. A nut 34 serves for setting the screw 32 so that the extent of pivotal motion of the member 27 about the shaft 18 can be controlled in this manner. A portion or element 35 of sound-dampening materialfor instance of the same type as discussed earlier-is preferably interposed between the head 33 and the member 27 (it may be carried by either one of them) to reduce noise.

The article to be conveyed is shown in broken lines and in fragmentary form in FIG. 4. Its direction of conveyance upwardly is indicated by the broken-line arrow 1a and it will be appreciated that as the article moves upwardly from below the member 27 it will engage the edge face 27b of the member 27 which extends into its path at an upward angle of inclination, whereby the member 27 will be pivoted in clockwise direction against the urging of the spring 29, thereby permitting the article 1 to move upwardly beyond the supporting face 27a. At this time the article 1 will be slightly above the supporting face 27a and the portion 28; as the lift means 3 subsequently descends from its upper position to its lower position the article will similarly descend into contact with the supporting surface 27a and the portion 28 and will be retained thereby, it being evident that as soon as the article 1 has originally cleared the upper edge of the edge face 27d, the member 27 will have been restored in counterclockwise direction by the spring 29 to the position which is illustrated in FIG.

Thus, the present invention achieves its purpose in a simple and highly reliable manner, and permits vastly increased throughput of the arrangement.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a conveying arrangement for conveying articles from a lower level to a higher level, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A conveying arrangement for conveying articles from a lower level to a higher level, comprising upright frame means having a lower terminal station, an upper terminal station, and at least one intermediate station between and equally spaced from both said terminal stations by a first distance; lift means guided in said frame means for alternate movement in a predetermined path between a lower position and an upper position by a second distance slightly greater than said first distance, said lift means having one lower and at least one upper transfer level which in said lower position are respectively located slightly below said lower terminal station and said intermediate station and in said upper position are respectively located slightly above said intermediate station and said upper terminal station; moving means for moving said lift means between said positions thereof; detent means including first detent means provided at the respective transfer levels and arranged for supporting articles to be conveyed during movement of said lift means to said upper position, and second detent means provided at the respective stations and arranged for permitting upward movement of articles supported by said first detent means during movement of said lift means to said upper position but for intercepting and retaining such articles at the respective stations in response to subsequent movement of said lift means to said lower position, said detent means comprising pluralities of sets of biased pawl units with each set being mounted at one of said stations or levels, respectively, and including a shaft and a double-armed pawl member mounted for pivotal movement on said shaft between a first position permitting upward movement of said articles and a second position in which one arm of said pawl member intercepts and retains such articles; abutment means provided with each of said pawl units and adapted to contact the respective other arms of an associated pawl member; and adjusting means engaging the respective other arm and having an engaging surface for engaging the same, and a cushioning portion of sound-dampening material provided on said engaging surface.

2. A conveying arrangement as defined in claim 1, wherein said pawl units are spring-loaded pawl units.

3. A conveying arrangement as defined in claim 1; and further comprising supply means for articles to be conveyed to said lower terminal station, and receiving means for receiving conveyed articles from said upper terminal station.

4. A conveying arrangement as defined in claim 3, said supply means and receiving means each comprising conveyor means for conveying articles to and away from the proximity of said lower and upper terminal stations, respectively; and said supply means and receiving means each further comprising transfer means for transferring articles from said supply means to said lower terminal station, and from said upper terminal station to said receiving means, respectively.

5. A conveying arrangement as defined in claim 4, said conveyor means of at least said receiving means being arranged for conveying said articles in an at least substantially horizontal path.

6. A conveying arrangement as defined in claim 1, wherein said shafts extend at least substantially normal to said path.

7. A conveying arrangement as defined in claim 1, wherein said sound-dampening material is a synthetic plastic material.

8. A conveying arrangement as defined in claim 1, said pawl members each having a recess, and said spring means comprising an expansion spring at least partly located in said recess and having an end abutting against a bottom wall of the same, said spring means further comprising a sleeve surrounding said spring partly received and guided in said recess and having an outer end urged by said spring into abutting engagement with said abutment means. 

1. A conveying arrangement for conveying articles from a lower level to a higher level, comprising upright frame means having a lower terminal station, an upper terminal station, and at least one intermediate station between and equally spaced from both said terminal stations by a first distance; lift means guided in said frame means for alternate movement in a predetermined path between a lower position and an upper position by a second distance slightly greater than said first distance, said lift means having one lower and at least one upper transfer level which in said lower position are respectively located slightly below said lower terminal station and said intermediate station and in said upper position are respectively located slightly above said intermediate station and said upper terminal station; moving means for moving said lift means between said positions thereof; detent means including first detent means provided at the respective transfer levels and arranged for supporting articles to be conveyed during movement of said lift means to said upper position, and second detent means provided at the respective stations and arranged for permitting upward movement of articles supported by said first detent means during movement of said lift means to said upper position but for intercepting and retaining such articles at the respective stations in response to subsequent movement of said lift means to said lower position, said detent means comprising pluralities of sets of biased pawl units with each set being mounted at one of said stations or levels, respectively, and including a shaft and a double-armed pawl member mounted for pivotal movement on said shaft between a first position permitting upward movement of said articles and a second position in which one arm of said pawl member intercepts and retains such articles; abutment means provided with each of said pawl units and adapted to contact the respective other arms of an associated pawl member; and adjusting means engaging the respective other arm and having an engaging surface for engaging the same, and a cushioning portion of sounddampening material provided on said engaging surface.
 2. A conveying arrangement as defined in claim 1, wherein said pawl units are spring-loaded pawl units.
 3. A conveying arrangement as defined in claim 1; and further comprising supply means for articles to be conveyed to said lower terminal station, and receiving means for receiving conveyed articles from said upper terminal station.
 4. A conveying arrangement as defined in claim 3, said supply means and receiving means each comprising conveyor means for conveying articles to and away from the proximity of said lower and upper terminal stations, respectively; and said supply means and receiving means each further comprising transfer means for transferring articles from said supply means to said lower terminal station, and from said upper terminal station to said receiving means, respectively.
 5. A conveying arrangement as defined in claim 4, said conveyor means of at least said receiving means being arranged for conveying said articles in an at least substantially horizontal path.
 6. A conveying arrangement as defined in claim 1, wherein said shafts extend at least substantially normal to said path.
 7. A conveying arrangement as defined in claim 1, wherein said sound-dampening material is a synthetic plastic material.
 8. A conveying arrangement as defined in claim 1, said pawl members each having a recess, and said spring means comprising an expansion spring at least partly located in said recess and having an end abutting against a bottom wall of the same, said spring means further comprising a sleeve surrounding said spring partly received and guided in said recess and having an outer end urged by said spring into abutting engagement with said abutment means. 